Hadith 3:889

Narrated Urwa:

Aisha said, "Buraira came to me and said, 'My people (masters) have
written the contract for my emancipation for nine Awaq ) of gold) to
be paid in yearly installments, one Uqiyya per year; so help me."
Aisha said (to her), "If your masters agree, I will pay them the whole
sum provided the Wala will be for me." Buraira went to her masters and
told them about it, but they refused the offer and she returned from
them while Allah's Apostles was sitting. She said, "I presented the
offer to them, but they refused unless the Wala' would be for them."
When the Prophet heard that and 'Aisha told him about It, he said to
her, "Buy Buraira and let them stipulate that her Wala' will be for
them, as the Wala' is for the manumitted." 'Aisha did so. After that
Allah's Apostle got up amidst the people, Glorified and Praised Allah
and said, "What is wrong with some people who stipulate things which
are not in Allah's Laws? Any condition which is not in Allah's Laws is
invalid even if there were a hundred such conditions. Allah's Rules
are the most valid and Allah's Conditions are the most solid. The Wala
is for the manumitted."